Methods of finishing reconstituted metal

ABSTRACT

METHODS ARE DISCLOSED FOR ROLLING RECONSTITUTED METAL, I.E., METAL OBTAINED BY SOLID STATE COMPACTION OF PIECES OF STEEL, INTO SHEET OR STRIP PRODUCTS WHICH POSSESS EXCELLENT FORMABILITY CHARACTERISTICS. HOT ROLLED PRODUCTS ARE PRODUCED BY A PREFERRED METHOD INCLUDIING THE STEPS OF ROLLING RECONSTITUTED METAL AT A FINISHING TEMPERATUE FROM ABOUT 1400*F. TO 1500*F. TO A FINAL THICKNESS WHICH IS NO GREATER THAN 20% OF THE ORIGINAL THICKNESS, HOT COLLING THE ROLLED PRODUCT WHILE AT A TEMPERATURE OF AT LEAST 1200* F., PREPERABLY AT LEAST 1270* AND ALLOWING THE COILED METAL TO COOL AND SELF-ANNEAL IN AIR. THE PREFERRED METHOD OF MAKING HOT ROLLED PRODUCTS FROM RECONSTITUTED METAL MAY INCLUDE A NOVEL PICKLING OPERATION WHEREIN THE ANNEALED, ROLLED METAL IS HEATED PRIOR TO BEING IMMERSED IN A HOT ACID BATH. DEEP DRAW QUALITY, COL ROLLED PRODUCTS ARE MADE BY COL ROLLING RECONSTITUTED METAL TO REDUCE ITS THICKNESS BY AN AMOUNT LESS THAN 50% AND THEN ANNEALING THE ROLLED METAL.

HOT ROLL FINISHING TEMP, "F

Filed July 21.

M. E. WHALEN ET AL 5 Sheets-Sheet 1 ZONE I ZONE 2 ZONE 3 I I I I l I I I 800 950 I000 I050 H00 H50 I200 I250 I300 I350 I400 I450 I500 F I G 2 COILING TEMPERATURE F OLSEN BUTTON TEST VALUES LEVEL AK QUALITY COMMERCIAL GAGE FK-Ll .0 INVENT OR. MARK E.WHALEN JOSEPH W. MALLECK ATTORNEYS Aug. 14, 1973 M. E. WHALEN EI'AL 3,752,710

METHODS OF FINISHING RECONSTITUTED METAL Filed July 21, 1971 5 Sheets-sheet 2 MARw wzxsaN I 5 6 JOSEPH w. MALLECK aw, mfizwaw ATTORNEYS Aug. 14, 1973 M. E. WHALEN ETAL 3,752,710

METHODS OF FINISHING RECONSTITUTED METAL 5 Sheets-Sheet 5 Filed July 21, 1971 FIG. 8

FIG

H69 HQ '0 INVENTORS BY MARK E.WHALEN J0 EPHW A LECK 14% we ATTORNEYS Aug. 14, 1973 M. E. WHALEN ETAL 3,752,710

METHODS OF FINISHING RECONSTITUTED METAL Filed July 21, 1971 5 Sheets-Sheet 4 INVENTORS FIG. |4MARK E.WHALEN JOSEPH w. MALLECK ATTORNEYS Aug. 14,1973 I M. E. WHALEN EFAI- 3,752,710

METHODS OF FINISHING RECONSTITUTED METAL Filed July 21, 1971 SheetsSheet 5 FIG. l5

INVENTORS MARK E. WHALEN BY JOSEPH W. MALLECK ATTORNEYS US. Cl. 148-12 28 Claims ABSTRACT OF THE DISCLOSURE Methods are disclosed for rolling reconstituted metal, i.e., metal obtained by solid state compaction of pieces of steel, into sheet or strip products which possess excellent formability characteristics. Hot rolled products are produced by a preferred method including the steps of rolling reconstituted metal at a finishing temperature from about 1400 F. to 1500 F. to a final thickness which is no greater than 20% of the original thickness, hot coiling the rolled product while at a temperature of at least 1200 F., preferably at least 1270 F. and allowing the coiled metal to cool and self-anneal in air. The preferred method of making hot rolled products from reconstituted metal may include a novel pickling operation wherein the annealed, rolled metal is heated prior to being immersed in a hot acid bath. Deep draw quality, cold rolled products are made by cold rolling reconstituted metal to reduce its thickness by an amount less than 50% and then annealing the rolled metal.

CROSS-REFERENCE TO RELATED APPLICATIONS Application Ser. No. 121,861 of Mark E. Whalen et al., filed Mar. 8, 1971 and entitled Apparatus and Solid State Method for Converting Small Pieces of Metal to a Workpiece. Application Ser. No. 122,110 of Mark E. Whalen et al. filed Mar. 8, 1971 and entitled Novel Apparatus and Solid State Method for Converting Small Pieces of Metal to a Workpiece.

SUMMARY OF THE INVENTION This invention relates generally to methods of finishing reconstituted metal, and more specifically to methods of finishing reconstituted metal to produce rolled products exhibiting excellent formability characteristics.

The invention is particularly concerned with methods for rolling reconstituted metal bodies, i.e., metal slabs obtained by the solid state compaction of discrete pieces of steel, into: 1) hot rolled products, (2) cold rolled non-killed products, and (3) cold rolled aluminum-killed products. Rolled products produced by the methods of this invention are characterized by good forming properties and can be made to have the qualities of conventionally produced drawing steel.

In a preferred method of producing hot rolled products, a dense ferrous body or slab obtained by the solid state compaction of pieces of steel is hot reduced by an amount such that the ratio of the original thickness of the body to its rolled thickness is at least 5:1 and is more preferably 7:1. The hot reducing step is performed with a finishing temperature which is at least 1400 F. and which preferably does not exceed 1550 F. The optimum upper limit of the finishing temperature is 1500 F. The hot rolled product is then air cooled from a temperature of at least 1200 F. to anneal the metal to a condition in which the formability characteristics, preferably as determined by the Olsen Button test, are at least equivalent to that of conventionally produced hot rolled steel. In the preferred process, the rolled sheet or strip is coiled while it is still hot following the hot reduction step, i.e., before the metal has cooled below a temperature of about 1270 F.

nited States Patent 0 3,752,710 Patented Aug. 14, 1973 "ice The immediate coiling of the sheet or strip traps the heat of hot reduction and results in self-annealing of the metal as it is cooled in air to room temperature. The self-annealing action which is obtained by hot coiling at a temperature of at least 1200 F., more preferably 1270 F. or higher, causes a desired amount of grain growth and also stabilizes the crystalline structure to relieve the stresses produced by previous working of the metal. The grain size resulting from the preferred hot coiling and selfannealing process is ASTM #8 or larger.

The invention also provides a novel procedure for pickling the hot rolled product obtained by the process generally described above. This pickling operation is accomplished by heating preferentially the scale and adjacent surface of the rolled product, as by infra-red heaters, prior to immersion in a hot acid bath. The preheating of the reconstituted rolled metal prior to immersion increases the efficiency of the pickling action.

In the preferred process of producing cold rolled nonkilled products, hot rolled reconstituted metal, such as obtained from rimmed steel scrap, is cold reduced by an amount which is preferably less than 50%. The cold reduction step is followed by a step of annealing the metal from a temperature of about 1330" F. or slightly less to enhance the drawing properties. The preferred method of cold reduction (50% or less) coupled with a high annealing temperature produces isotropic formability, i.e., good formability characteristics in all directions of the rolled sheet or strip. The grain size of the cold rolled metal is on the order of ASTM #6-7.

The invention further contemplates the production of rolled sheet or strip products which exhibit both high strength and good ductility. This is accomplished by hot finishing at a temperature in the range of from 1500 F. to 1600 F. To obtain a product in which the grains at the surfaces of the metal are relatively large and the grains in the center are relatively small. The surface grains are recrystallized after cold reduction by annealing at a temperature in the range of from about 900 F. to 1200 F.

In accordance with the processes of the invention, both the hot rolled and cold rolled products may be subjected to a skin rolling operation in order to improve surface finish. The skin rolling operation is preferably carried out to effect a reduction in thickness of from about 5 to 10% Deep drawing characteristics are obtained in an aluminumkilled cold rolled product by a technique in which the reconstituted metal is heated to obtain solution conditions during slabbing or strip finishing, whereby aluminum and nitrogen are redissolved in austenite. Controlled reprecipitation of aluminum nitrite particles takes place following finishing processing of the reconstituted metal to provide a crystallographic orientation of the optimum character for drawing operations.

A fuller understanding of the methods of this invention will the had from the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing Olsen Button for different gauge rolled products produced by the methods of this invention;

FIG. 2 is a chart relating hot finishing temperature and coiling temperature;

FIGS. 3 through 8 are microphotographs of hot rolled specimens produced by difierent hot finishing and hot coiling temperatures; and

FIGS. 9 through 15 are microphotographs of specimens of hot and cold rolled products.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS For purposes of clarity, the specification is divided into the following sections:

(I) Production of reconstituted metal by solid state compaction (II) Production of hot rolled products (III) Production of cold rolled non-killed products (IV) Production of cold rolled aluminum-killed products (I) Production of reconstituted metal by solid state compaction As generally described above, the present invention provides methods of processing reconstituted metal bodies, i.e., metal slabs obtained by the solid state compaction of pieces of steel, into rolled strip or sheet products. Processes and apparatus for producing reconstituted metal slabs or bodies are disclosed in the above-referenced applications Ser. Nos. 121,861 and 122,110, and the disclosures of these applications are incorporated herein by reference.

As generally described in the referenced applications, reconstituted metal is made from discrete pieces of steel, e.g., steel scrap. In an initial stage of the solid state compaction process, discrete steel pieces of random sizes are mechanically compressed into bales. The baling of the random sized scrap is carried out in a manner which causes the pieces to become physically intertwined and interlocked to form a cohesive body. The cohesiveness of the bale body contributes to the inhibition of force dissipation when the bale is subsequently impacted.

In a subsequent stage of the process, the bales are heated in a furnace preparatory to a hot forming operation. The bales may be first heated to a temperature of from 1000 F. to 1200 F. in an inert atmosphere and then heated in a reducing atmosphere at a temperature in the range of from 1200" F. to 2000 F. The heated bales are then forged in a hot harmonic press to reduce each bale to a solid mass of steel. The bale is constrained against lateral movement in the press as it is forged and therefore against lateral dissipation of the forging forces. The peripheral confinement of the bale and the high impacting forces cause molecular migration and welding of the scrap into a homogeneous, unitary slab of reconstituted steel.

The subsequent sections of this specification are directed to novel methods of roll finishing the slab of steel into strip or sheet products having good formability characteristics.

(II) Production of hot rolled products Reconstituted metal produced in the foregoing manner by solid state compaction is generally characterized by relatively small grain size. In order to produce hot rolled sheet or strip having good formability characteristics, it

is important to anneal the metal subsequent to hot rolling to obtain a grain size of about ASTM #8 or larger. This is preferably accomplished by immediate hot coiling following the step of hot rolling. Immediate hot coiling means that the rolled strip or sheet is coiled while it is still hot following hot reduction and before it has had time to cool below a desired annealing temperature. The hot coiling operation traps the residual heat in the coil so that the metal self-anneals as it slowly cools in air from the coiling temperature. This self-annealing action promotes the desired amount of grain growth and results in a more formable product. Although too large a grain is recognized to be undesirable because of surface orange peel effects during any subsequent forming operations, an unduly large grain size does not result from hot coiling reconstituted metal because of the fact that it has an initially small grain prior to rolling. Hot rolled reconstituted metal which has been subjected to hot coiling exhibits a typical grain size of ASTM #8.

The coiling temperature of the metal should be above 1200 F., and more preferably at least 1270 F. The coil is preferably wrapped so that the metal in the coil will hold at a temperature in the range of from about 1200 F. to 1270 F. for approximately 2 hours before cooling to room temperature by radiation. If the coiling temperature is in the desired range of from 1200 F. to 1270 F., there will be a sufiicient amount of grain growth and recrystallization of the metal during self-annealing regardless of the hot finishing temperature. The annealing operation will also have the effect of permitting the atoms of metal to reassemble into stable positions, thereby relieving the mechanical stresses set up by working.

Although some degree of grain growth and recrystallization results from self-annealing regardless of the hot finishing temperature, optimum strength and formability characteristics have been obtained by hot reduction with a finishing temperature which is at least 1400 F. and does not exceed 1550 F., more preferably 1500" F. The slight amount of working during hot reduction below 1500 F. amplifies the typical small grain size of the reconstituted metal prior to being hot finished, and promotes considerable grain growth upon self annealing to relieve stresses and achieve a stable crystalline structure. Satisfactory hot rolled products can be produced at a finishing temperature above 1600 F. provided that the process includes the hot coiling step described above, although the resulting product will not have the optimum formability characteristics obtained by the preferred finishing temperature range of from 1400 F. to 1500 F. It is believed that finishing temperatures above 1600 F. adversely effect the grain size and grain pattern. Finishing temperatures of from 1500 F. to 1600 F. are normally avoided because of the mixed grain size which results although finishing temperatures in this range may be utilized in certain cold rolling applications as will be presently discussed.

In the preferred method, the slab or body of reconstituted metal is hot rolled to reduce its cross-sectional thickness by an amount such that the ratio of the Original thickness to the rolled thickness is at least 5:1 and is more preferably at least 7:1. Good results have been obtained by a reduction of thickness in the ratio of 10: 1. The preferred minimum reduction of 5 :1 during hot reduction is desired to achieve bonding of the reconstituted metal so that it may be subjected to subsequent forming operations, such as deep drawing. The minimum reduction ratio depends upon the amount of solid reduction previously received during slabbing and these values are based upon a slab previously receiving about 20% of solid reduction.

To provide a good surface finish on the hot rolled products so that they can be used in applications where smoothness is important, the present invention contemplates adding a temper or skin pass to achieve a limited amount of cold reduction. The temper pass primarily Works the surface grains of the metal without working the center section and provides good levelling characteristics. The preferred amount of cold reduction of the temper pass should be in the range of from about /2% to 10% and is preferably in the range of from 5% to 10%. If the temper or skin pass is applied with proper grit blasted rolls, it is possible to obtain a surface finish having a mean peak depth of about 100 to 125 microinches. Although this is higher than conventional cold rolled matte finishes, a subsequent coating of vapordeposited zinc and an over-coating of chrome-chromate will smooth out the finish so that it from about to micro-inches.

Another important advantage resulting from employing a skin or temper pass on hot rolled reconstituted metal becomes apparent if the metal was hot rolled at a high finishing temperature followed by coiling below 1270 P. so that a fine grain results. The fine grain would normally increase yield elongation by an amount such that the metal could not be used in many drawing operations. Skin or temper rolling according to the invention overcomes this disadvantage.

is in the range of 6 Hot rolled metal is conventionally pickled and oiled tured. Certain of the samples were subjected to the Fukui when sold as a hot rolled product. A conventional pickling cup test which involves both stretching and bending of operation generally involves moving the hot rolled strip the specimens. In conducting the Fukui test, the specimens material through an acid bath at an elevated temperature, were cut into circles of varying diameter depending upon e.g., 170 F. to 220 F. It has been found that the efli- 5 the gauge of the metal and a steel ball was pressed upciency of the pickling operation can be enhanced by wardly through a conically shaped opening until the speciheating preferentially the scale on the hot rolled reconmen ruptured. The final diameter of the specimen was stituted metal prior to immersion in the acid bath. This then compared to the initial diameter and the value chartmay be accomplished by any suitable manner, as by infraed. Preferred results are .79 or less for hot rolled steel. red heaters. Another advantage of heating the reconsti- (E) A generally accepted criteria for measuring the tuted metal prior to immersion is that it permits acid deep drawing qualities of steel is the R value. This is the baths of lower concentration to be used without sacrificratio of strain in the plane of the sheet to the strain ing efficiency. through the sheet and is expressed as The following Table I tabulates parameters and test 1 W W results of eight hot rolled specimens produced from re- R= L constituted metal. The nominal chemical compositions 10g To/T of the eight specimens are listed in Table II. where W0 is the width of the specimen after straining, To

TABLE I Sample number 1 2 3 4 5 6 7 8 Type of scrap .035 Al? .035 Al? .036 AK..." .035 AK .034 rin1 .035 rim"... .035 nm AK Cleaning preparation Flame 800 Flame 800. Flame 800.- Dirty Sand blast" Sand blast Sand blast" Sand bla t, Reheating furnace temperatures 2 3 2 3 2 3 3 2 3 0 2,35

Reheating furnace atmosphere Hot roll finish temperature... Coiling temperature- Hot rolled grain size- Hardness (Rockwell 3)- Cleanliness rating Carbide rating Tensile-45 to rolling (k.s.i.) Tensile longitudinal (k.s.i.) Tensile-transverse to rolling (k.s.i.) Elongation (percent) 2 Olsen button test (inch). Hot reduction ratio Finish gage R=In (WOW) in the direction of rolling.

In(To/T) Carbon deposits.

TABLE II is the thickness of the specimen before straining, and T S Sn Ni Cu M0 or Si A1 18 the thickness of the specimen after straining. These values depend upon the orientation of the piece of steel .1 2 .2 .02 .002 .015 .01 .05 8% 8 8 8 .002 .015 .03 relat1ve to the direction of rolling. Therefore, the more 1107 m5 important R value is called R, whereby specimens are i 32 2; 3 2%: a? g: '3; tested in the direction of rolling (R0), at to the direc- :8l :002 :02 :02 1009 10;. :02 :01 45) n at 90 to the ir i n f r l in .010 .002 .02 .02 .007 .015 .03 .02 45 (R These values are added together and divided by 4 .010 .002 .02 .02 .002 .015 .01 .05 according to the equation The following is a summary of definitions and terms R R 13 4. R 45 used in the metallurgical evaluations of the specimens as appearing in Table I.

(A) cleanliness rating cleanliness for the Specimens Typical values for aluminum killed deep drawing steels was determined by a Comparison with Standard ASTM shoulddbe 1.4 to 1.6, and typical values for cold rolled specimens in chart form. Representative pieces were cut xg ig sg g gi lengthwise or parallel to the direction of rolling and were d f e Ger am speclmerls were P l' h d The sam le ieces were then inspected under a need. mm i'econstltuted metal scrap conslstlng of AK 9 e at 100px i nification and were Compared (aluminum-killed) deep drawn quality steel. Other speci- 3 g chart A nugmerical rating of was used mens were produced from reconstituted metal consisting to indicate the size of oxide or silicate inclusion. A prefix ggg gg fi {Li All f i had a th.1ckneS of is used to distinguish between oxide or silicate (A=oxide, a 6 Scrap ma ena was re const lmted Into Dsilicate)- and a suflix is used to indicate distribution a a accordmg theprocedures descnbed m the first j inclusions e g T=thick etc sect1on of the spec1ficat1on which included heating prior 0 (B) Grain si2e specimens were subjected to a suitto forging m E i and/or reducing atmosphere a a j temperature 0 a out 2350 F. In the formation of the apleptchant g l g m graglrboundanes' The gram slab, the scrap material was compacted to 100% density sue 18 mp d C e d t ed and was additionally compacted 20-60% beyond solid arbne rat1ng. ar 1 grill mg We; ermm, density. Certain of the samples were heated in an oxidizby mICI C P egamrrlatlorld at magm F 5 g ai ing atmosphere for the hot reduction step and others were g g :22gpg g gzgfgi giggl were Ju ge sub ected to an inert atmosphere. In all cases the Spegie as g mens were heated for hot reduction to a tem eratur of (D) Formability evaluation-This was pr1nc1pally dep e about 2300 F. Certain specimens were given a 5:1 retermined by the Olsen Button test which consists of pressduction during hot rolling and other specimens were given ing a Steel ball of predetermined diameter upwardly a 10:1 reduction during rolling. As discussed above, it is through a fiat Specimen clamped about its Perimeter In believed that the minimum preferred reduction ratio of accorda h Conventional Procedures, the test Was 5:1 is desired to achieve sufficient bonding to permit the carried out at a specific deformation rate and the ultimate material to be subjected successfully to subsequent formheight of deformation was measured when the metal ruping operations.

The cold coiling temperatures resulted from a test procedure wherein certain samples were individually allowed to air cool rapidly to 1200 F., this being equivalent of coiling a continuous strip of metal after it has cooled to a temperature below 1200 F. The hot coiling temperatures resulted from a test procedure in which specimens at a temperature above 1200 F. were stacked among other hot specimens so as to retard cooling, this being the equivalent of coiling a hot rolled strip to retain the heat of hot reduction.

Samples l 2 and 3 of Table 1 represent hot rolled strips which have been coiled cold, i.e., below a temperature of 1200 F. The structure of these specimens was charac terized by ultra-fine distribution of carbides. When the metal Was hot coiled as represented by samples 4 through 8, a degree of carbide agglomeration took place so that the carbon could be characterized as occurring as finely scattered pearlite With the beginning of the formation of some massive carbide particles.

The greater degree of agglomeration of carbon in the form of pearlite and massive carbide particles promotes increased formability because of the fact that the carbon is removed from the grain boundaries. Accordingly, sam ples 4 through 8 demonstrate the advantages of hot coiling, especially when coupled with a finishing temperature which is no greater than about 1500 F. (sample 6). Another factor contributing to the good forming properties of the hot rolled products is the carbon content of the metal. As will be seen from Table II, the carbon level of the reconstituted metal is exceedingly low.

It will be seen that the tensile properties do not necessarily correlate directly with hardness values. Nevertheless, there is a general indication that the lowest tensile values which usually correlate with good formability were obtained with the specimens having the largest grain growth obtained from hot coiling coupled with a finishing temperature of no greater than 1500 F. It appears from these tests and others that hot rolled, reconstituted metal scrap may exhibit a higher level of strength than was previously considered desirable for good forming properties. In particular, it now appears that the conventional concept that good forming characteristics depend upon a tensile strength below 40,000 p.s.i. is not true for reconstituted steel.

Another observation was the non-directionality of the properties of strength and formability. These properties were good in all directions so that the hot rolled products can be classified as isotropic in this regard. This is believed due to the fact that the scrap pieces were randomly oriented in the baled body and the fact that the general grain and crystal orientation of the starting material is not significantly changed by subjection to solid state compaction. The good non-directionality of strength and formability properties is confirmed by the R values of samples 4 through 8 which are in the range of from 1.2 to 1.4.

One of the most important tests of formability is the Olsen Button test. Samples 6, 7 and 8 which were processed with a finishing temperature between about 1400 F. to 1500 F. followed by hot coiling at a temperature above 1270 F. generally produced the best Olsen Button test values. While the samples produced with hot finishing temperatures above 1500 F. coupled with cold coiling achieved some degree of formability, the formability was not at the level obtained with hot coiling. As a whole, the Olsen Button test values obtained with the test samples indicate that the formability of hot rolled reconstituted metal made according to the preferred method of this invention is at least equivalent to that of hot rolled sheet and strip produced from hot melted, aluminum killed quality steel. This is made particularly evident by FIG. 1 which is a graph of the Olsen Button test values of Table I plotted against the gauge of the particular sample. (FIG. 1 also shows Olsen Button test values of cold rolled samples which are discussed in the next section of this specification.)

Reference is now made to FIGS. 2 through 8 which further demonstrate the desirable effects on grain size and grain pattern obtained by following the practice of the present invention of immediate hot coiling in order to self-anneal the reconstituted metal. FIG. 2 is a chart relating finishing temperatures and coiling temperature. The chart of FIG. 2 is divided generally into three zones depending upon the hot finishing temperature. Zone 1 has a minimum temperature of about 1600 F. at coiling temperatures up to 1270 F. The minimum temperature of zone 1 increases with coiling temperatures above 1270 F. Hot finishing temperatures in zone 1 generally produce a uniformly equiaxed but moderately small grain structure. This is evidenced by the microphotographs of FIGS. 3 and 4. FIG. 3 is a microphotograph of a rolled specimen which was made with a hot finishing temperature above 1600 F. followed by coiling at a temperature below 1270 F. FIG. 4 is a microphotograph of a specimen which was rolled at a hot finishing temperature above 1600 F. followed by hot coiling above a temperature of 1270 F. In each instance, it will be seen that the grain structure is relatively small, e.g. ASTM #9.

Zone 2 of FIG. 2 has a minimum hot finishing temperature of about 1500 F. Finishing temperatures in zone 2 produce a mixed sized grain structure such that the rolled metal has larger grains at the surface and generally smaller grains in the center. FIGS. 5 and 6 are microphotographs of specimens which were rolled with a finishing temperature in zone 2. The grain structure shown in FIG. 5 resulted from a process including a coiling temperature below 1270 F. and the grain structure shown in FIG. 6 resulted from a hot coiling temperature above 1270 F. As discussed above, finishing temperatures in zone 2 are normally avoided for hot rolled products because of the resulting mixed grain structure, although such a grain structure may be advantageous in producing cold rolled products exhibiting high strength and high ductility, as will be presently discussed.

Zone 3 of FIG. 2 lies below the second zone and includes finishing temperatures below about 1500" F. Cold working begins to occur substantially throughout the entire cross-section of metal rolled at finishing tempera tures in this zone. FIG. 7 is a microphotograph of a specimen prepared by rolling with a temperature below 1500 F. followed by cold coiling below 1270 F. The elongated and distorted grains which occur substantially throughout the entire cross-section of the specimen shown in FIG. 7 are fixed by the cold coiling temperature. FIG. 8 is a microphotograph of a specimen prepared according to the preferred process of the invention by a finishing temperature below 1500 F. followed by hot coiling above 1270 F. The self-annealing action obtained by hot coiling produces a substantially uniform, stabilized crystalline structure of relatively large grains, ASTM #8, which result in excellent formability characteristics in all directions.

(III) Production of cold rolled non-killed products In producing cold rolled reconstituted metal from nonkilled scrap, e.g. rimmed steel scrap, it is preferred to (1) coil at a temperature below 1270 F., e.g. about 1050 F. following hot reduction and (2) provide less cold roll reduction than conventional in order to cooperate with the already high degree of cold working previously imparted to the initial scrap material forming the body of reconstituted metal. The amount of cold rolled reduction which is conventional to attain good drawability is at least In the preferred process of this invention the amount; of cold rolled reduction is less than 50%.

The ability to cold roll reconstituted metal by an amount less than 50% while obtaining a product exhibiting good drawing characteristics is premised on the use of an annealing operation following cold working. Batch annealing is preferable. The batch annealing operation should be carried out with a slow heat-up and cool- 10 EXAMPLES The following Table III sets forth four examples of cold rolled products produced from reconstituted rimmed steel scrap. The nominal chemical compositions of the examples in Table III are given in Table IV.

TABLE III Type of scrap .035 rim .034 Tim 034 rim 036 rim. Cleaning preparation- Sand blast. Sand blast Sand blast Sand blast Reheating furnace temp Inert Hot roll finish temp Coiling temp Hot rolled grain size Hardness-hot rolled (R Hot rolled cleanliness Hot rolled carbide rating Very fine carbide in zone. Hot rolled tensile L Hot rolled tensile 45 Hot rolled tensile, transver Hot rolled Olsen te Hot rolled gage Percent cold reduction Percent 1st pass Total passes Annealing temp Annealing time (hours Annealing heat-up rat Grain size Cold rolled annealed hardness. Cold rolled cleanliness Cold rolled surface rating Cold rolled carbide rating. Cold rolled tensile L (k.s.i.) Cold rolled tensile 45 (k.s.i.) Cold rolled tensile, transom (k.s.i.)

surface.

Few stringers under Full anneal massive...

Clean. lean and sound Clean and sound.

A Clean and sound C Cold rolled elongation Cold rolled button test.

Cold rolled Fukul Value.

Cold rolled thickness 014 0. 25 23 down rate in order to control grain size and growth. If TABLE Iv continuous annealing is to be employed after cold work- P S Sn Ni Cu M or Si Al ing, such as is used in connection with coated product 006 014 3 lines, the temperature of coiling following hot reduci I006 I022 88 :3; g; :88? i l? .gi .8

tion ma be raised above the referred maximum leve .014 .023 .003 .02 .02 .004 .015 y p .006 .023 .005 .02 .02 .008 .015 .01 .01

of 1050 F. in order to produce a larger grain size. While a high coiling temperature is not preferred in producing cold rolled products because it effects directionality of the forming properties, such directionality may be tolerated if the continuously annealed product is not subjected to complex drawing. Continuous annealing subjects steel sheet to fast heat-up rates, e.g. seconds, and involves holding the steel at the annealing temperature for about 15 minutes. This short dwell and quick heat-up does not permit the metal to achieve a grain pattern which will be as stable as can be obtained by batch annealing unless the grain pattern has been previously improved by hot coiling.

The annealing temperature for reconstituted metal follows the same pattern as that for conventional metal. For example, low carbon steel should be annealed slight- 1y below 1330 F. When the metal is heated to a temperature slightly above 1330 F., the degree of agglomeration of carbides will be good but will not be optimum for drawability. However, if the carbon content of the reconstituted metal is extremely low, e.g. about .Ol% or lower, the annealing temperature range is not so critical and may vary from between about 1250 F. to 1370 F.

Surface finishing for cold rolled reconstituted metal is effected by the speed of rolling and the severity of metal reduction in the beginning passes. Optimum finish can be obtained by holding the surface velocity of the rolls in the range of from 600 to 1500 feet per minute with less than about 18% reduction in the first pass. At speeds of from 1500 to 3500 feet per minute and with a heavy first pass of or more reduction, the likelihood of surface springers is increased by the excessive cold work. Normally, a cold rolled product produced in accordance with this invention will not require a skin or temper pass.

1375 F. FIG. 13 is a microphotograph of sample 10 after it was completed as a hot rolled product but before cold reduction. FIGS. 14 and 15 are microphotographs of sample 10 following cold reduction. Sample 10 was sub- JECIfid to a lower percentage of cold reduction, i.e., 40%, and was produced with a higher annealing temperature of 1350 F.

The results of these tests indicate that isotropic formability, that is, formability in all directions, is enhanced when the starting metal is substantially rimmed quality steel and is cold rolled and annealed. It is also indicated that hot coiling above 1270 F. should be avoided in the production of cold rolled products. This is because hot coiling will cause carbide agglomeration to take place before cold working. Such carbides will be elongated by cold working to promote directionality of the forming properties. When the metal is given a more con ventional hot finish at higher temperatures of about 1050 F., the carbides will remain finely divided in the rolled product. i

(IV) Production of cold rolled aluminum-killed products Conventional processing techniques for producing aluminum-killed, deep draw steel include hot finishing temperatures above 1600 F., low coiling temperatures of 1 050" F. or lower, 50 to 70% cold reduction, and continuous heating to a soaking temperature of about 1050 F. which is then stepped up to annealing temperature of about l 350 F. in order to effect softening. These steps are used with metals having about .02% to 08% by weight 1 1 aluminum, .02% to .04% by weight manganese, and about .002% to 008% by weight nitrogen.

The discovery of the present invention is that if solution conditions are established in the scrap material during slabbing or during subsequent roll finishing so that aluminum and nitrogen are redissolved in austenite, controlled reprecipitation of aluminum nitride particles will take place following roll finishing so as to provide a crystallographic orientation which is best for formability. In carrying out this process the amount of cold-rolling should be less than 50% in order to obtain the proper elongated grain pattern. This reduced amount of cold reduction is desired because of the heavy cold working of the scrap material prior to being used in making reconstituted metal.

Strip steel products are conventionally furnished in either a ductible, annealed condition with recrystallized grain structure or in an unannealed condition exhibiting high strength with a cold worked metallurgical structure. The present day methods of finishing thus involve a compromise of ductility for high strength or vice versa, although it is known that there are many applications which desirably require the use of both high strength and ductility.

In accordance with the present invention, it is possible to improve both strength and ductility. This is accomplished by employing hot finishing temperatures in the range of from 1500 F. to 1600 F. so as to produce a mixed grain structure as described previously. In this structure the strip metal has relatively large grains at its surfaces and smaller grains at its center. It has been discovered that a recrystallized surface will provide an increase in ductility in comparison with unannealed, highstrength, conventionally produced strip material, while the unrecrystallized center will provide an increase in strength. The combination of the strength and surface ductility can be best achieved using low carbon rimmed steel scrap wherein the nitrogen content is maintained relatively low. A method for producing reconstituted metal having low carbon and nitrogen levels is taught in the copending application of applicants, Ser. No. 164,789 filed July 21, 1971, and entitled Apparatus and Solid State Method for Converting Small Pieces of Steel to a Workpiece.

In order to achieve the desired qualities of strength and ductility, the mixed grain structure produced by hot finishing temperatures in the range of from about 1500 F. to 1600 F. is subjected to a controlled heat treatment so as to preferentially recrystallize the surface grains and allow for suitable grain growth, while leaving the center of the metal substantially unrecrystallized. The cold reduced, rimmed steel surface is recrystallized by annealing at a temperature in the range of from 900 F. to 1200 F. for a period of time sufficient to substantially recrystallize the surface grains, but insufiicient to substantially effect recrystallization of the center of the metal. This may be accomplished by batch annealing the metal in coil form for a period of from 4 to 12 hours at the recommended temperature. If continuous annealing is employed, it can be performed as rapidly as 1000 to 2000 feet per second. Continuous annealing should be performed at the same temperature of from 900 F. to 1200 F. Preferential surface recrystallization of rimmed steel, rolled strip is a function of time and temperatures. Lower temperatures can be employed in batch annealing because the strip is maintained at the annealing temperatures for a relatively long time, whereas higher temperatures are required for continuous annealing because of the shorter holding times. More specifically, rimmed steel strip can be preferentially recrystallized at its surfaces by batch annealing at 900 F. for a period of from 4 to 30 hours or by continuous annealing at 1200 F. in as little as 30 to 40 seconds.

Many modifications and variations of the invention will be apparent to those skilled in the art in the light of the foregoing disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than as specifically shown and described.

What is claimed is:

1. A method of producing a rolled product from a slab of reconstituted ferrous metal comprising the steps of:

5 (a) hot rolling the ferrous slab at a finishing temperature of at least 1400 F.,

(b) arranging the rolled metal to trap the heat of hot rolling before the metal has cooled below about 1200 F.,

(c) and permitting the rolled and arranged metal to cool slowly and self-anneal to a condition in which the formability characteristics determined by the Olsen Button test are at least equivalent to conventionally produced hot rolled strip.

2. A method as claimed in claim 1 in which the rolled metal is arranged to trap the heat of hot rolling by placing it in a coil.

3. A method as claimed in claim 2 in which the rolled metal is coiled at a temperature of at least 1270 F.

4. A method as claimed in claim 1 in which the slab is hot rolled at finishing temperatures of from 1400 F. to 1500 F.

5. A method as claimed in claim 1 in which the slab is rolled at a finishing temperature of at least 1600 F.

6. A method as claimed in claim 1 in which the slab is rolled to reduce its cross-section thickness by an amount of at least 80%.

7. A method as claimed in claim 1 in which the slab is rolled to reduce its cross-sectional thickness by an amount such that the ratio of the original thickness of the slab to the rolled thickness is at least 7: 1.

8. A method of converting pieces of steel into a rolled product comprising the steps of:

(a) providing a solid body obtained by solid state compaction of pieces of steel,

(b) hot rolling said body to reduce its cross-sectional thickness by an amount such that the ratio of the thickness before rolling to the thickness after rolling is at least 5:1,

(c) said hot rolling step being carried outwith a finishing temperature of at least 1400 F.,

(d) hot coiling the rolled metal before it cools below a temperature of about 1200 F.,

(e) and maintaining the hot rolled body in coiled form so that the metal self-anneals during cooling.

9. A method as claimed in claim 8 in which the rolled metal is hot coiled at a temperature of at least 1270 F.

10. A method as claimed in claim 9 in which the rolled metal is coiled so that the metal is held at a temperature of from 1200 F. to 1270 F. for about two hours.

11. A method as claimed in claim 8 in which said hot rolling step is carried out with a finishing temperature in the range of from 1400 F. to 1550 F.

12. A method as claimed in claim 8 in which said hot rolling step is carried out with a finishing tempera- 55 ture of at least 1600 F.

13. A method as claimed in claim 8 in which the coiled metal is allowed to cool and self-anneal to a condition ingracterized by an average grain size of at least ASTM 14. A method as claimed in claim 8 in which the coiled metal is allowed to cool and self-anneal to a condition having a formability characteristic determined by the value it to be about 1.1 to 1.3.

15. A method as claimed in claim 8 including the subsequent step of temper rolling the metal after cooling to effect a reduction in thickness of from about 5% to 10%.

16. A method as claimed in claim 8 including the subsequent steps of pickling the annealed metal by prefer- 70 ential heating of the surface of said metal with any scale thereon and then immersing the metal in a hot acid bath.

17. A method of converting pieces of steel into a hot rolled product comprising the steps of:

(a) providing a dense ferrous body obtained by solid state compaction of pieces of steel,

(b) hot rolling said body to reduce its cross-sectional thickness in an amount such that the ratio of the original thickness of said body to the final thickness after rolling it at least 7: 1,

(c) carrying out said hot rolling step at a finishing temperature in the range of about 1400" F. to 1550 F,

(d) arranging the hot rolled metal into a coil before the metal cools below a temperature of about 1270 F.,

(e) said rolled metal being coiled so that the metal is held at a temperature above 1200 F. for about two hours,

(f) and allowing said coil to slowly cool and selfanneal.

18. A method of converting pieces of non-killed steel into a cold rolled product comprising the steps of:

(a) providing a dense ferrous body obtained by solid compaction of pieces of rimmed steel,

(b) hot reducing the cross-sectional thickness of said body,

(c) carrying out said hot reducing step at a finishing temperature of at least 1400 F.,

(d) thereafter cold rolling the metal to further reduce its cross-sectional thickness by an amount of 50% or less,

(e) and annealing the cold rolled metal at temperatures of from 1250 F. to 1370" F. for a period of from 10 to 25 hours.

19. A method as claimed in claim 18 in which the cold rolled metal is annealed at a temperature in the range of from about 1325 F. to 1350 F.

20. A method as claimed in claim 18 including the step of coiling the metal after the step of hot reduction, the metal being coiled at a temperature below 1200 F.

21. A method as claimed in claim 20 in which the metal is coiled at a maximum temperature of about 1050 F.

22. A method of producing drawing quality steel comprising the steps of:

(a) providing a dense vferrous body obtained by solid state compaction of pieces of rimmed steel,

(b) hot reducing the cross-sectional thickness of said body with a finishing temperature above 1400 F.,

(c) thereafter cold rolling said metal to further reduce its cross-sectional thickness by an amount less than 50% of the thickness prior to cold rolling,

(d) and annealing the cold rolled metal at a temperature in the range of from about 1325 F. to 1350 F. for a period of from about 10 to hours.

23. A method as claimed in claim 22 in which the cold rolling step is carried out with a roll surface velocity of .from 600 to 1500 feet per minute.

24. A method as claimed in claim 23 in which the metal 14 is cold rolled in a plurality of passes with the maximum reduction in thickness in the first pass being about 18%.

25. A method of producing aluminum-killed, deepdrawing quality steel comprising the steps of:

(a) providing a ferrous ody obtained by solid state compaction of pieces of aluminum-killed, deepdrawing quality steel having an aluminum content in the range of from about .02% to 08% by weight and a nitrogen content in the range of from about .002% to 008% by weight,

(b) said body being at least 95% solid,

(c) hot reducing the cross-sectional thickness of said body with a finishing temperature of at least 1400 F.,

(d) thereafter cold rolling said body to .turther reduce its cross-sectional thickness by an amount no greater than 50%,

(e) and annealing the cold rolled body at a temperature of from about 1300 F. to 1370" F.

26. A method of producing reconstituted strip steel characterized by high strength and good ductility comf prising the steps of:

(a) providing slab obtained by solid state compactio of pieces of steel,

(b) at least the outer portion of said body being rimmed quality steel,

(0) hot reducing the cross-sectional thickness of said body with a finishing temperature in the range of from about 1500 F. to 1600 F.,

(d) thereafter cold rolling the metal to .further reduce its cross-sectional thickness,

(e) and annealing the cold rolled metal at a temperature in the range of from 900 F. to 1200 F.

27. A method as claimed in claim 26 wherein the cold rolled metal is batch annealed in coil form for a period of from 4 to 12 hours.

22;. A method as claimed in claim 26 in which said annealing step is carried out continuously at a temperature of about 1200 F.

References Cited UNITED STATES PATENTS 1,491,392 4/ 1924 Graham 226 2,435,511 3/1948 Rice 75226 3,270,409 9/ 1966 Grant 75226 3,334,408 8/1967 Ayers 264-111 3,359,100 12/1967 Claus et a1 75226 WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 75226 

